1. Field of The Invention
The invention relates to loudspeaker systems. In particular, the invention relates to loudspeaker systems that enhance the reproduction of sub-bass frequencies.
2. Description of the Prior Art
The major obstacle in accurately reproducing bass frequencies is that of providing consistent acoustic loading of the driver cone at lower frequencies, that is to say frequencies having long wavelengths. In air, the acoustic length of a 20 Hz signal is 56 ft. Therefore, the cone of the driver must have a constant acoustic impedance presented to it throughout the entire wavelength of the signal if distortion and signal loss are to be avoided. This occurs when the cone moves but does not linearly pressurize the adjacent air mass as a signature of the electrical signal input. This requirement contributes directly to the cost of true low frequency sound reproducers, because bass frequencies below 100 Hz become more difficult to produce as the driver dimensions and enclosure volume become small relative to the wavelength. Moreover, room acoustics makes bass systems even more difficult to integrate sonically without expensive hardware and impractical and costly interior modifications.
In the early 1950""s the acoustic suspension enclosure for loudspeakers was developed which allowed bass response to be extended. When combined with a smaller enclosure and a driver with a heavy long throw mechanism, a low frequency driver substituted efficiency for low bass extension. The bass reflex enclosure was introduced earlier and popularized in the 1960""s by Theile and Small to produce more efficient high Q bass response (boomy) and was easy to manufacture.
From those early days up to the present, virtually all successfully marketed loudspeakers use some variation of such enclosures.
In an effort to satisfy the general population, the audio industry has concentrated on bass magnitude (High Q) rather than quality (critical damping), and as a result, such convention will only support cost effective strategies for volume production.
Accordingly, the bass reflex enclosure system dominates in popularity as it can achieve a balanced pressure dynamic operation at high levels. Thus, it is the most efficient speaker design for its size and least costly to manufacture. Reflex systems are designed to produce the lowest frequencies at the box resonance as output falls at a rate of 24 db/oct below that frequency. This is caused by close coupled acoustic phase cancellations that occur in conjunction with the unloading of the driver and port simultaneously.
In addition, signal purity is compromised in several ways with reflex systems as two distinctive radiating sources are producing the same signals at opposing phases. The system is (periodic)resonant by design and therefore unstable in its damping characteristics. Proper T/S alignment is a must and some loss in transient response is still unavoidable. The rapid roll off (24 db/oct) below resonance and Q variations makes cost effective designs unnaturally boomy in sound quality as the compromises impact overall realism.
Over the years, there have been many attempts to design and build an efficient and useful bass reflex speaker system.
For instance, U.S. Pat. No. 3,684,051 shows a bass reflex loudspeaker cabinet incorporating speakers and a corrugated cardboard acoustic duct. However, since the duct is formed of cardboard, the overall sub-bass frequency response of the speaker is impaired.
U.S. Pat. No. 3,690,405 shows a loudspeaker having a pair of acoustic cavities coupled by a port aperture. The port aperture is included in one of the cavities, and the second cavity may include dampening. The speaker is mounted in the first cavity. Unfortunately, this structure is complicated in design and requires expensive manufacturing procedures.
U.S. Pat. No. 4,714,133 shows a loudspeaker having an enclosure, a cone driver, ports, and an acoustic resonator. The resonator defines front and rear cavities, and serves as the focal point for all radiated or vibration induced audio energy. The ports serve as pressure relief valves to support driver activation of the resonate screen, as a means for matching the driver and the enclosure low frequency resonance, and as a sound dispersion device around the enclosure to create the illusion that sound is not driver oriented but is emanating externally of the enclosure. Nevertheless, sub-bass frequencies are not accurately reproduced by this loudspeaker.
U.S. Pat. No. 5,514,841 shows a reflex compression valve-divided chamber speaker cabinet having a ported speaker baffle chamber, a chamber divider, polyester batting, and a tuned free-flow air slot. The speaker operates on the principle of controlling both compressed and decompressed air flow within the ported speaker baffle chamber by means of the chamber divider, which controls air flow past the divider to form a valve combined with the slot. Unfortunately, this speaker cabinet is complicated in structure and design, and does not offer significant bass response.
In general, none of the previously discussed loudspeakers are suitable for efficient reproduction of sub-bass frequencies, that is to say, frequencies below 100 Hz, without compromising on the quality of low bass signal reproduced. None of these designs emphasizes to shorten the wavelengths of sub-bass frequencies for proper loading of the driver.
Therefore, a need exists for a speaker capable of reproducing sub-bass signals without compromising overall acoustic quality or imposing an undesirable restriction on either the listening environment, or the physical size and decorative appearance of the speakers.
Accordingly, it is an object of the present invention to provide a full range loudspeaker which offers the beneficial attributes of bass reflex operation while eliminating the adverse effects.
It is another object of the present invention to provide a sub-bass loudspeaker that is efficient, has low cone mass and offers low excursion at its lowest frequencies.
It is a further object of the present invention to provide a sub-bass loudspeaker capable of shielding the driver from signals reflected by the walls of the listening room, or signals which normally alter the radiation resistance and frequency response of the driver cone.
It is yet another object of the present invention to provide in one enclosure, a full range loudspeaker system such as a bass reflex speaker system, a subwoofer system or an auxiliary audio/video product (TV, radio, etc.).
It is a further object of the present invention to provide a loudspeaker having diminished physical vibration from the speaker cabinet.
It is yet a further object of the present invention to provide a sub-bass loudspeaker that is physically small, attractive and cost effective.
Further objects and advantages of the invention will become more readily apparent in view of the following detailed description of the preferred embodiments of the invention.